The nuclear power industry long has been engaged in a multitude of studies and investigations seeking improvement in the stamina and reliability of the materials and components forming a reactor-based power system. One such investigation has been concerned with intergranular stress corrosion cracking which heretofore principally has been manifested in the water recirculation piping systems external to the radiation intense reactor core regions of nuclear facilities. Typically, the piping architecture of these external systems is formed of a stainless steel material.
Generally, the studies referred to above have determined that three factors must occur in coincidence to create conditions that promote intergranular stress corrosion cracking. One factor is a sensitization of the metal such as stainless steel, for example, by chromium depletion at grain boundaries. Chromium depletion at grain boundaries may be caused by heat treatment in the course of normal processing of the metal or by welding and the like procedures. A second factor is the presence of tensile stress in the material. A third factor is the oxygenated normal water chemistry environment typically present in a boiling water reactor. This latter environment is occasioned by any of a variety of oxidizing and corrosive species contributed by impurities in reactor coolant water. Corrosive species are generally impurity ions such as chlorides or sulfates. Therefore, monitoring of the corrosive species would be helpful in the study and control of intergranular stress corrosion cracking.
Electrical conductivity probes are used in liquids having at least a slight degree of electrical conductivity and are able to measure increases or decreases in that conductivity. A voltage is applied between separate electrodes immersed in the fluid. Small changes in current are then detected between the electrodes, as the concentration of conducting species changes. The electrodes must be well insulated from one another and arranged to provide a conducting path between them of fixed dimensions with paths for stray leakage currents minimized. As the concentration of corrosive species in the reactor water of a boiling water nuclear reactor increases, the conductivity of the water will increase. Therefore, conductivity probes measuring increases in the conductivity of reactor water can be used to detect increases in the concentration of corrosive species in that water.
While the conductivity probe can measure the conductivity of the water, certain parts of the conductivity probe must be insulatively sealed from intrusion of the water. An internal conductor and the connection between the conductor and an electrode in the probe must be sealed and protected from the reactor water to prevent electrical interference, shorting and corrosion of the conductor.
Conductivity probes developed for use in high-pressure and high temperature liquid environments have been configured with combinations of metal housings and ceramic insulators combined with mechanical interference or washer type seals made of polymeric or soft metallic materials. These structures have performed adequately in the more benign and essentially radiation free environments of, for example, recirculation piping in nuclear reactors.
Over the recent past, investigators have sought to expand the conductivity probe monitoring procedures to the severe environment of the fluid in the vicinity of the reactor core itself for the purpose of studying and quantifying the effect of corrosive species on stress corrosion cracking. Within the reactor core, conductivity probes can be mounted in specially designed small cross section tubing. Such tubing is located among the fuel elements in the reactor core, and is used to house various monitoring devices such as neutron detectors. As a result, these tubes are known as local power-range monitor tubes.
Thus, the conductivity probes are located in the severe environment of the fluid in the reactor core having a typical high temperature of 274.degree. C., pressure of 1,000 psi, and radiation of 10.sup.9 rads per hour gamma and 10.sup.13 rads per hour neutron. Conductivity probe structures of earlier designs are completely inadequate for this reactor core environment, both from a material standpoint, and with respect to the critical need to prevent leakage of radioactive materials to the ambient environment of the reactor. For example, the polymeric seals used in most conductivity probes cannot withstand intense radiation with the result being failure of the probe and leakage of radioactive materials.